A complex network of signaling pathways controls the fundamental physiological activities of a cell, such as growth, migration and survival. Hence, perturbations of these pathways serve as the molecular basis underlying a wide range of human diseases, including cancer. A thorough understanding of the molecular mechanisms by which altered cellular signaling processes drive these diseases will greatly facilitate our ability to trat these conditions. In breast cancer, one of the most frequently dysregulated signaling pathways is the phosphoinositide 3-kinase (PI3K)/Akt cascade. Frequent mutation of the oncogene PI3K or inactivation of the tumor suppressor PTEN leads to hyperactivation of Akt, which promotes tumorigenesis through its downstream effectors. There are three isoforms of Akt (Akt1, Akt2 and Akt3). Our lab, among others, has demonstrated that Akt isoforms have non-redundant functions in the regulation of tumor progression. However, there is a paucity of knowledge regarding the mechanisms by which specific Akt isoforms signal to control these phenotypes. Newly published data from our lab supports an exclusive role for Akt2 in breast cancer cells with inactive PTEN. The main objective of this proposal is to determine the mechanism by which Akt2 isoform-specificity is conferred in PTEN-inactive breast cancer. The central hypothesis driving this proposal is that Akt2 isoform-specificity in PTEN-inactivate breast cancer is conferred by intrinsic molecular determinants that regulate phosphorylation of isoform-specific substrates controlling tumorigenesis. The outlined experiments will: (i) determine the oncogenic phenotypes of PTEN-inactive breast cancer cells exclusively regulated by Akt2 using in vitro cellular assays to assess the effect of Akt2-specific silencing on proliferation, growth, survival and invasive migration; (ii) identify and characterize novel Akt2-specific substrates in PTEN-inactive breast cancer cells using a SILAC global mass spectrometry phospho-proteomic screen; and (iii)determine whether molecular determinants on Akt2 define Akt2 isoform-specificity in the context of PTEN-inactivation using chimeras of swapped domains of Akt1 and Akt2 and performing in vitro cellular assays to rescue phenotypes associated with Akt2-silencing in PTEN-inactive breast cancer cells. The proposed research is novel given our poor mechanistic understanding of Akt isoform-specific signaling, particularly in the context of specifi PI3K-pathway mutations. The results of this work will fill a critical gap in our understanding of Akt isoform-specificity and have important implications for the development of clinically-relevant, isoform-specific inhibitors. Such inhibitors would provide a potential therapeutic opportunity to regulate the pro-tumorigenic activities of Akt2 in PTEN-inactivate breast cancer. Furthermore, identification of Akt isoform-specific substrates that regulate tumor progression will provide a se of novel targets with therapeutic potential. Collectively, these approaches will contribute to our long-term goal of tailoring therapeutic strategies to treat breast cancer patients with hyperactivated PI3K/Akt signaling.